Marble Goes Yellow

The common problem of yellowing in marble especially the white varieties such as Carrara . Most of us know that one of the main causes is iron.

So I will outline how this mechanism works. However there are many other reasons why a marble can turn yellow and these are not as well known or documented hence I will outline these as well. Iron is the most well known but least understood cause of yellowing.

It can be caused by both internal and external sources.

External sources include iron bearing minerals in the substrate, installation materials, surrounding fixtures, water supply as well as building rubbish such as abandoned nails etc.

Internal sources are the solubleminerals naturally occurring in the stone. It is the internal source that requires explanation. Most of the internal iron occurs as iron sulphide bearing minerals.

The most common is Pyrite and also to a lesser extent marcasite. The process starts by exposing the iron to oxygen. The main source of oxygen is water. However other oxidisers include acids as well as bleaches. Although we are all aware of the problems associated with the use of acid on marble and limestone many people do not know why household bleaches are not recommended. Bleach, as an oxidiser will set off the iron creating yellowing of the stone.  Once the iron has oxidised the iron sulphide oxidises to iron sulphate and then to iron hydroxide (we know it as rust). The release of sulphur as sulphate, in the presence of water, converts to either sulphurous acid or even sulphuric acid.  So the result of all this chemistry is that we get a yellow stone and, if the sulphate/sulphuric acid phase occurs, a stone that can etch itself due to it’s own acid production. How about the other less documented causes for turning marbles yellow because the yellowing is not always rust or iron.

  1. Incorrect Maintenance: Accumulation of dirt especially oil based in the worn surface of a stone can look yellow. Dirty mops and poor cleaning chemicals will exasperate this. The use of bleach as mentioned above promotes oxidation of any iron. Therefore clean regularly with clean mops and good ph neutral cleaners such as AQUA MIX STONE CLEANER.
  2. The incorrect polishing of stone can turn it yellow. If the stone is polished using steel wool any particles left on the surface will create rust. This is even worse if the pads are used on very wet stone. If acidic polishing compounds are not correctly neutralised this can crate a yellowing. Crystallization materials can also create the problem if not totally neutralised and are especially problematic if the stone is too wet.
  3. Wax build up or other topical sealers such as urethanes can turn yellow. The solution is simple here – use Aqua Mix coatings as they DO NOT YELLOW. Coatings can also appear to turn yellow as they trap dirt in their worn surface. So it is prudent to clean coatings regularly and re-coat when necessary.
  4. Wooden substrates can create yellowing. Some timbers used in flooring have natural gums and resins as well as chemicals used in their treatment that can migrate towards the stone’s surface causing discoloration.
  5. Dyes from rubber mats or the jute backing of some carpets will turn a marble yellow. Some of the older cutback adhesives used for carpet and vinyl flooring can turn a stone yellow. These types of adhesives should be removed and AQUA MIX SEALER & ADHESIVE REMOVER does a fine job
  6. Some epoxies (now used in grouts and ever increasingly as adhesives) can discolour stone. Most are two part and it is the unreactive part that when not mixed completely with the reactive hardener can migrate into the stone in moist conditions causing a yellow-brown stain.
  7. Lastly UV in combination with an unstable sealer coating and bleaches and oxygenated water will turn a marble yellow. So as you can see it is not only iron that can turn a marble yellow. When you are next on a problem job investigating yellowing do take into account the other variables, as it is most often a combination of things that create the discoloration. Also as water and oxygen are two of the most important catalysts for iron staining do remember to recommend the use of an Aqua Mix Sealer (as this will reduce water absorption into the stone) and the use of our non-bleach cleaners.

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